Don't overload FreeListElement for become.

runtime/vm/isolate_reload.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/isolate_reload.h"
 
 #include "vm/become.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
 #include "vm/dart_api_impl.h"
@@ skipping 390 matching lines @@
   RawClass** local_saved_class_table =
       reinterpret_cast<RawClass**>(malloc(sizeof(RawClass*) * saved_num_cids_));
 
   Class& cls = Class::Handle();
   UnorderedHashSet<ClassMapTraits> old_classes_set(old_classes_set_storage_);
   for (intptr_t i = 0; i < saved_num_cids_; i++) {
     if (class_table->IsValidIndex(i) &&
         class_table->HasValidClassAt(i)) {
       // Copy the class into the saved class table and add it to the set.
       local_saved_class_table[i] = class_table->At(i);
-      if (i != kFreeListElement) {
+      if (i != kFreeListElement && i != kForwardingCorpse) {
         cls = class_table->At(i);
         bool already_present = old_classes_set.Insert(cls);
         ASSERT(!already_present);
       }
     } else {
       // No class at this index, mark it as NULL.
       local_saved_class_table[i] = NULL;
     }
   }
   old_classes_set_storage_ = old_classes_set.Release().raw();
@@ skipping 543 matching lines @@
     : ObjectVisitor(),
       handle_(Object::Handle()),
       owning_class_(Class::Handle()),
       owning_lib_(Library::Handle()),
       code_(Code::Handle()),
       reload_context_(reload_context) {
   }
 
   virtual void VisitObject(RawObject* obj) {
     // Free-list elements cannot even be wrapped in handles.
-    if (obj->IsFreeListElement()) {
+    if (obj->IsFreeListElement() || obj->IsForwardingCorpse()) {

[Cutch, 2016/06/02 14:09:53]

This seems like a common enough pattern that we might want a utility method.

[rmacnak, 2016/06/02 17:57:06]

Done.

       return;
     }
     handle_ = obj;
     if (handle_.IsFunction()) {
       const Function& func = Function::Cast(handle_);
 
       // Switch to unoptimized code or the lazy compilation stub.
       func.SwitchToLazyCompiledUnoptimizedCode();
 
       // Grab the current code.
@@ skipping 200 matching lines @@
         ASSERT(!super_cls.IsNull());
         super_cls.AddDirectSubclass(cls);
       }
     }
   }
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart